It shouldn't matter if the blade is different because you are not using the new key in the door lock or the ignition, you are just holding it near the car and using the remote part of it. Likewise with the immobiliser part as you're not trying to use that new key in the ignition so it doesn't need to be able to deactivate the immobiliser. You will need to get those bits done to be able to use the key though.
I think the answer to the third question there is you should be pressing the buttons on the NEW remote, so it recognises the signals from that new remote and associates them with locking and unlocking the car.
We still haven't got our
problem resolved and are looking at something like £300 or more for a new central convenience unit